The United States and Japan will step up their defence cooperation to deal with the threat from nuclear-armed North Korea as tensions in East Asia remain high, officials from the two allies said on Thursday.

New Labor leader to be named on Oct 13

Labor's new leader will be announced on October 13 after a ballot of the caucus and grassroots members of the party.

Two senior caucus members, Bill Shorten and Anthony Albanese, will contest the ballot.

But nominations will remain open until 5pm (AEST) on Thursday.

The ALP national executive decided on Monday to open the grassroots ballot to all members who were financial as of September 7, rather than impose a limit on members who had been on the books for two years or more.

The members ballot will open on September 24 and close on October 9.

Caucus members will meet at 4pm (AEST) on October 10 to cast their ballots and the final result will be announced on October 13.

"This is a good thing for Australian democracy, a good thing for the Labor party, to invite so many more people to share with us the important decision about who should be our alternative prime minister leading our party over the next term," acting Labor leader and former treasurer Chris Bowen told reporters in Canberra on Monday.